Center to limb variation of transition region Doppler shift in active regions

Abstract

Studying Doppler shifts provides deeper insights into the flow of mass and energy in the solar atmosphere. We perform a comprehensive measurement of Doppler shifts in the transition region and its center-to-limb variation (CLV) in the strong field regions ($|\text{B}|\ge$ 50 G) of 50 active regions (ARs), using the \ion{Si}{4} 1394~Å line recorded by the Interface Region Imaging Spectrometer(IRIS). To locate the ARs and identify strong field regions, we have used the magnetograms obtained by the Helioseismic and Magnetic Imager (HMI). We find that in strong field regions, on average, all the ARs show mean redshifts ranging between 4{--}11~ km/s, which varies with ARs. These flows show a mild CLV, with sizable magnitudes at the limb and substantial scatter at the mid-longitude range. Our observations do not support the idea that redshifts in the lower transition region (T $<\sim$ 0.1 MK) are produced by field-aligned downflows as a result of impulsive heating and warrant alternative interpretation, such as downflow of type-\rm{II} spicules in the presence of a chromospheric wall created by cooler type-\rm{I} spicules.

Accepted for publication in the Astrophysical Journal on 16th January 2023

References in corpus (5)

• Key Aspects of Coronal Heating
• Flows and Motions in Moss in the Core of a Flaring Active Region: Evidence for Steady Heating
• Spectroscopic Observations of a Coronal Loop: Basic Physical Plasma Parameters Along the Full Loop Length
• Fan Loops Observed by IRIS, EIS and AIA
• On Doppler shift and its Center-to-Limb Variation in Active Regions in the Transition Region